1. Field of the Invention
The present invention relates to a display control device, controlling a display via a display driver.
2. Description of the Related Art
Conventionally, in devices such as cellular phones equipped with an LCD (Liquid Crystal Display) panel, the LCD panel is directly controlled by a display driver (hereinafter, simply referred to as a “driver”). In the basic configuration, the driver is controlled by a host equipped with a CPU (Central Processing Unit). However, recent cellular phones and the like have increased and sophisticated functions, and as a result, loads on hosts have become excessive.
For this reason, a configuration wherein the display controller takes part of the functions from the host has been increasing. The display controller is provided between the host and the driver, and is a device to perform part of the processing which the host has conventionally performed when the host outputs a control signal to the LCD panel, in place of the host. More specifically, the display controller takes functions to receive instructions from the host, to perform predetermined processing such as changing image size, and to output image data to the driver.
As for devices such as cellular phones equipped with a display controller in this way, when the display controller is not in use to provide power savings, the display controller is placed into a halt state (a state that a clock in the display controller is in a halt state). On the other hand, even when the display controller is in a halt state in such a case, a small amount of data may need to be discretely transmitted to the driver from the host. For example, a case that updating an indicator, indicating a radio wave receiving state, by only one scale in the whole screen on a display of cellular phone.
In such a case, in order to access the driver from the host with the display controller in a halt state, a bypass function is provided to transmit a signal through the display controller. FIG. 6 is a diagram showing a conventional display control device 100 equipped with a bypass function.
In FIG. 6, a host 110 is coupled to a display controller 120 by a chip select signal line (hereinafter, referred to as “CS signal line”) and a sub-chip select signal line (hereinafter, referred to as “SCS signal line”). Furthermore, the display controller 120 is coupled to a driver 130 by a data signal line and a chip select signal line (hereinafter, referred to as “FPCS signal line”). Therefore, the host 110 can output a signal to the driver 130 only through the display controller 120.
As for the configuration shown in FIG. 6, when the display controller 120 is not in a halt state and a CS signal (chip select signal) is outputted to the display controller 120 from the host 110, the display controller 120 generates a signal (FPCS signal) corresponding to the CS signal in a computing unit 121 and outputs it to the driver 130.
On the other hand, when the display controller 120 is in a halt state, even when the CS signal is inputted from the host 110, the FPCS signal is not generated because the display controller 120 is in a halt state, therefore, the signal from the host 110 is not outputted to the driver 130. Thus, a selector 122 is provided in the display controller 120 and the selector 122 is set to a non-bypass mode (a state in which a bypass is not performed) in states other than the halt state, and the signal from the computing unit 121 in the display controller 120 is outputted to the driver 130.
When an access to the driver 130 from the host 110 is required in the halt state, the host 110 switches the selector 122 to the bypass mode (a state in which a bypass is performed) to pass the SCS signal (sub-chip select signal) outputted from the host 110, as it is, and outputs it to the driver 130.
However, according to the above-described scheme, the use of two CS signal lines (the chip select signal line and the sub-chip select signal line) is required so that the host 110 controls the display controller 120. Because the number of CS signal lines provided for the host 110 are limited, it is preferable that similar control is performed by using fewer of the CS signal lines.
Thus, outputting the above-described CS signal and SCS signal to the display driver 130 from the host 110 by using one CS signal line is considered. FIG. 7 is a diagram showing a configuration of a display control device 200 outputting the CS signal and the SCS signal by one common CS signal line. In FIG. 7, the host 210 is coupled to the display controller 220 by a CS signal line and outputs a CS signal and a SCS signal to a display controller 220 by the CS signal line. Furthermore, the display controller 220 is coupled to a driver 230 by a data signal line and a FPCS signal line. Except for the above-described configuration, each function unit in the display control device 200 is similar to the corresponding unit in the display controller 100 shown in FIG. 6.
As for the display control device 200 shown in FIG. 7, when a switching signal to the bypass mode is outputted by the CS signal line during the non-bypass mode, a selector 222 in the display controller 220 is switched to the bypass mode, and after that, the SCS signal outputted from the host 210 is outputted to the driver 230.
On the other hand, when switching to the non-bypass mode during the bypass mode, the host 210 outputs a signal for switching the selector 222 in the display controller 220 from the bypass mode to the non-bypass mode to the display controller 220. However, at this time, the selector 222 is in the bypass mode. Therefore, the signal for switching may be bypassed and outputted to the driver 230 due to a time lag when the selector 222 is switched. When the switching of the bypass mode and the transmission of the CS signal are performed by one common chip select signal line, the signal for switching the bypass mode becomes noise, which causes an LCD panel or the like to malfunction.
A related drive control of an LCD panel is disclosed in Japanese laid-open patent publication No. H7-239674.
The related art described in Japanese laid-open patent publication No. H7-239674 concerns solving a problem of signal processing of a driver, controlling an LCD panel, but it does not solve the above-described problem on a display controller. Thus, it has been difficult to realize a bypass function by using fewer of the chip select signal lines without causing a malfunction in the display controller.
Therefore, an object of the present invention is to realize a bypass function for a display controller by using fewer of chip select signal lines without causing a malfunction.